Description: (Applicant's Description) We are proposing to have a central animal CORE for breeding the transgenic mice. All mice required by the four investigators on this program project would be provided by the CORE. Dr. Jaffee would be the CORE leader since she has been breeding the colony and has been successful. The CORE would be responsible for breeding and distributing the mice. Individual investigators will be responsible for housing mice that are used in experiments. There are four specific aims of this CORE: 1) Breeding the transgenic mice to the numbers required for studies proposed in all four projects; 2) Mating female mice once between the age of 12 and 14 weeks to enhance expression of HER-2/neu which in turn will enhance the rate of spontaneous tumor development; 3) Quarterly testing of 5-10 mice in the colony for expression of the HER-2/neu transgenic; 4) Distribution of mice to all four investigators. This CORE is essential for the conduction of the in vivo experiments proposed by all four project leaders.